IC technology making use of an organic semiconductor device has been attracting much attention. The main advantages of the organic semiconductor device resides in that it can be produced at a low cost and that a flexible resin can be used as a substrate. Owing to these advantages, the organic semiconductor device is expected to be used in circuits comprising a plastic substrate, display drive circuits for electronic tags and displays, and memories.
In general, the organic semiconductor device is composed of a substrate, insulating layer, electrode, and organic semiconductor layer. Particularly, a device composed of a gate insulating layer, gate electrode, source electrode, drain electrode, and organic semiconductor layer is used as a thin film field effect transistor (FET).
In an FET device including an organic semiconductor as a semiconductor layer, when voltage (gate voltage, Vg) applied to the gate electrode is changed, the amount of charge at the interface between the gate insulating layer and the organic semiconductor layer becomes too large or too small. As a result, a source/drain current (Id) flowing from the source electrode to the drain electrode through the organic semiconductor varies, thereby making switching possible. Therefore, the FET device fulfills a device function.
In fact, a high-performance organic semiconductor device has been obtained by applying a solution of an organic semiconductor compound such as polyalkylthiophene compound or polythienylenevinylene compound (refer to Assadi A., et al., “Field-effect mobility of poly(3-hexylthiophene)”, Appl. Phys. Lett., vol. 53, pp. 195 (1988), Fuchigami H., et al., “Polythienylenevinylene thin-film transistor with high carrier mobility”, Appl. Phys. Lett., vol. 63, pp. 1372 (1993), and Japanese Patent Application Laid-Open No. H10-190001).
However, when a flexible resin substrate is used, it is greatly inferior to a silicon or glass substrate in smoothness and flatness. In addition, when electrodes are formed by an inexpensive printing method, they are greatly inferior in surface smoothness and flatness. Therefore, when the insulation properties of the gate insulating layer are unsatisfactory, a leak current from the source electrode to the drain electrode becomes large.
When an organic semiconductor device is provided on a flexible resin substrate, constituents such as a gate insulating layer and organic semiconductor layer on the substrate must be produced at low temperatures of 200° C. or less. The reason for this is that when a device provided on a resin substrate is exposed to a high-temperature atmosphere, the resin substrate is softened or deteriorated.
For example, Shimoda et al. fabricated an FET including an insulating layer made of polyvinyl phenol (PVP) and electrodes and an organic semiconductor layer all of which were formed by ink jet printing (refer to Shimoda, et al., “Organic Transistor Manufactured by Ink Jet Printing”, Oyo Buturi, vol. 70, pp. 1452 (2001)). Meanwhile, Veres.B. J., et al. fabricated an FET including an organic semiconductor layer made of polytriallylamine formed on an insulating layer having a low dielectric constant (refer to Veres B. J., et al., “Low-k insulator as the choice of dielectrics in organic field-effect transistors”, vol. 13, pp. 199 (2003)). Since a thermoplastic resin is used as a material for forming the insulating layer in both of the above prior arts, the obtained FET's have high processability but involve problems with solvent resistance and heat stability. Therefore, they are not suitable for forming multiple layers and it has been difficult to form a thin layer showing sufficiently high insulating properties.
Although an FET including an insulating layer made of polyimide which is generally known to have high insulating properties has been reported, when an attempt is made to form a polyimide insulating layer from a precursor, high temperatures of 250° C. or more is required and it is therefore difficult to form an FET on a resin substrate.
Meanwhile, Z. Bao., et al. fabricated an FET by coating a polyimide insulating layer formed by baking at a low-temperature, a π-conjugated polymer active layer made of regioregular poly(3-hexylthiophene), and a source/drain electrode layer formed from an aqueous dispersion of a conductive polymer sequentially on a polyethylene terephthalate (PET) sheet having an indium tin oxide (ITO) gate electrode and drying them (refer to Z. Bao, Y. Feng, A. Dodabalapur, V. R. Raju, and A. J. Lovinger, “Chem. Mater.” 9, 1299 (1997)). Since it is considered that the surface of an ITO film formed on PET like the gate electrode used in the above prior art has low smoothness and the polyimide film formed by baking at a low temperature has low density, it has been difficult to form an insulating layer.
All of the above insulating layers do not show high insulating properties on a plastic substrate or an electrode having low smoothness and high mobility when an FET is formed.